1. Field of the Invention
The present invention relates to a probe for a scanning microscope which uses a nanotube as a probe needle thereof and more particularly to a probe for a scanning microscope in which the nanotube and a cantilever are made electrically continuous by means of a palladium covering film so that a voltage can be applied across the nanotube and sample.
2. Description of the Related Art
So as to image the structure of a sample surface by means of an atomic force microscope (abbreviated as “AFM”), a probe needle that is caused to contact or closely approach the sample surface and extract a signal is required. Conventionally, cantilevers made of silicon or silicon nitride in which a protruding portion (called a “pyramid portion”) is formed on the tip end of the cantilever have been known as such probe needles. Conventional cantilevers are manufactured using micro-fabrication techniques such as lithography, etching or the like. Such cantilevers sense the inter-atomic force of the sample surface with the tip end of the protruding portion; consequently, the imaging precision is determined by the sharpness of the tip end of the protruding portion. Accordingly, semiconductor working techniques that mainly include an oxidation process and an oxidation film etching process have been used for sharpening the tip end of the protruding portion that constitutes the probe needle. However, there are limits to the miniaturization that can be achieved even in electric current semiconductor working techniques; accordingly, there are also physical limits to the sharpness of the tip end of the above-described protruding portion. Meanwhile, carbon nanotubes have been discovered as a novel carbon structure. Such carbon nanotubes combine the most superior conditions as AFM probe needles.
In this area, H. Dai et al. have reported an AFM probe in which a carbon nanotube is bonded to the tip end of the protruding portion of a cantilever (see NATURE, Vol. 384, p. 147, 1996). The probe of these researchers was a break-through probe; however, this probe showed a tendency for the carbon nanotube to fall off of the protruding portion. In order to solve this weak point, the inventors of the present application developed fastening methods in which a carbon nanotube is firmly fastened to the protruding portion of a cantilever. The results of this development were disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. 2000-227435 and 2000-249712.
In the above-described first fastening method, the base end portion of the nanotube is irradiated with an electron beam so that a coating film is formed, and the nanotube is fastened to the protruding portion of the cantilever by being coated by this coating film. The second fastening method is a method in which an electric current is passed through the base end portion of the nanotube, so that the base end portion of the nanotube is fastened to the cantilever protruding portion by fusion.
As described above, commercially marketed cantilevers are produced using semiconductor working techniques, and the material of such cantilevers is silicon or silicon nitride. While silicon is a semiconductor, silicon nitride is an insulator. Accordingly, even if a conductive carbon nanotube is fastened to the protruding portion of such a cantilever, it is difficult to apply a voltage across the nanotube probe needle and the sample, or to cause an electric current to flow through the probe needle, since the cantilever itself does not possess conductivity.
In cases where the probe does not possess conductivity, this means that the use of the probe is severely restricted. More specifically, in conventional methods, the sample surface shape is merely detected via the amount of mechanical deformation of the nanotube; information relating to physical properties such as the mechanical characteristics of the sample surface and the like cannot be obtained. In cases where the probe possesses conductivity, a voltage can be applied across the probe and the sample, and an electric current can be caused to flow, so that physical properties such as the distribution of electrical resistance and the like can be measured. However, in cases where the probe does not possess conductivity, such voltage application and electric current flow are impossible, so that the electrical operation of the nanotube is impossible, thus limiting the effectiveness of the probe. For example, in a probe in which electrical operation is impossible, limits arise in terms of the operability in depositing atoms, moving atoms and extracting atoms on the sample surface. It appears that the working of samples by such manipulation of atoms will be a basic 21st century technology comparable to bio-technology. The inability to perform electrical operations on a nanotube leads to a restriction of the future possibilities of the probe itself.
Accordingly, in Japanese Patent Application Laid-Open (Kokai) No. 2002-162336, the inventors of the present application developed a probe for use in a conductive scanning microscope. In this patent, a technique is proposed in which a conductive covering film is formed on a cantilever, and the nanotube and conductive covering film are caused to be electrically continuous. For example, the conductive covering film is formed by vacuum evaporation, ion plating or sputtering of a metal.
However, carbon nanotubes consist of a carbonaceous material, and there has been almost no research on metal materials that can be tightly bonded to carbon nanotubes. Accordingly, if the wrong metal material is selected, the carbon nanotube may insulated from the metal coating film so that the electrical continuity is interrupted, or the conductivity may be insufficient.